With the increasing popularity, scale and complexity of web applications, design and development of web applications are becoming more and more difficult. However, the current state of their design and development is characterized by anarchy and ad hoc methodologies. One of the causes of this chaotic situation is that different researchers and designers have different understanding of web applications. In this paper, based on an explicit understanding of web applications, we present a unified architecture model of web applications, the four-view model, which addresses the analysis and design issues of web applications from four perspectives, namely, logical view, data view, navigation view and presentation view, each addressing a specific set of concerns of web applications. The purpose of the model is to provide a clear picture of web applications to alleviate the chaotic situation and facilitate its analysis, design and implementation.